Stimulus%20Presentation%20and%20Image%20Acquisition

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Stimulus Presentation and Image Acquisition

• Valentina Petre

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How do we Design an Experiment

• Block design

Event related
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Block design
Very good statistical power Good for all or
none phenomena (example visual, or acoustic
stim) Good for an initial experimental probe
pilot study (exceptions) The optimal block
length 14 to 20 s Do not depend much on the
assumption of a certain hemodynamic function
Subject might anticipate the stimulus
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Event related
Multiple trials presentation in one run
Optimum time 16s Close trials 4s still can be
separated You must assume linearity Depend
on the model of the hemodynamic function
Reduced in sensitivity vary the inter trial
interval
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Event Related stimulus presentation
desynchronized from
scanner acquisition
Slice i
Stim on
Frame aq
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Protocols available
128 x 128 64 x 64 Mosaic
Resolution Signal to Noise Time betw. Acq. Max of slices for 128 frames Time to acquire one slice Delay time Excitation order high 2.3 x 2.3 mm low long (10 min) 16 slices with 3.5s TR 120-130ms 140-150ms low 5 x 5 mm high short 27 slices with 3 s TR 100ms 140ms
Protocol
Prop
Interleaved
Ascending
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Small voxel volume, low signal
Large voxel, high signal
Air tissue interface
Tissue
Air
The field inhomogeneity is large, thus it will
cause large phase dispersion across the voxel.
The signal is reduced.
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How to avoid intravoxel dephasing

• Reduce the slice thickness. The longest dimension
  of your voxel should not be parallel with the
  direction of field variation
• Orient the slices oblique
• For spin echo sequences, the effect is smaller

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D Bo
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Equipment available

• 1.5 T Vision Siemens scanner head
  coil surface coil
• Head holder device ear
  protection nose piece
  bite bar
• Mirror and mirror stand
• Mouse joy stick
• Trigger box
• Projector cables
  RGB VGA
  screen
• Physiological monitoring

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Accessories
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MR compatible mouse
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Penetration panel
Inside the scanner room
Outside the scanner room
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Screens
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Types of stimulation used at the MNI

• Visual
• Acoustic
• Pain
• Somatosensory
• EEG

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Programs used for stimulus presentation

• GLstim available on the SGI (R. Hoge )
• Media Control Function on PC (Windows) Pierre
  Ahad
• SuperLab http//www.superlab.com
• Make your own.

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Parameters for the protocol to be used
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Artifacts
Patient wearing a metal studded belt
Braces
http//www1.stpaulshosp.bc.ca/stpaulsstuff/MRartif
acts.html
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RF Artifacts
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Safety issues

• Implants
• Pacemaker
• Metallic part left after surgery, or metallic
  fragments in their eyes
• Belts
• Tools (screw drivers, scissors, pens)
• Credit cards, bus pass.
• Certain type of makeup, tattoos
• Avoid loops

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Safety issues
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Safety issues contd..